1. Field of Invention
The present invention relates to a method and apparatus for modifying the surface properties of organic and inorganic polymer materials such as film and fabric, woven and non-woven.
2. Description of the Prior Art
One of the many utilities for meltblown polymer web is as a wet cell battery plate separator. The base polymer compound is impervious to the electrolyte. The meltblown, non-woven fabric structure is ion permeable if the surface thereof is thoroughly wetted by the electrolyte. Unfortunately, this latter requirement of wettability is not an inherent characteristic of most commercial polymers such as nylon, polypropylene, polyethylene and poly(ethylene terephthalate).
Although meltblown webs of these polymers are currently used as battery plate separators, wettability is achieved chemically by means of surfactants. This process not only generates hazardous industrial waste but produces a product of limited utility life.
Wettability is also a desirable property for tissue and cloth used to wipe or clean the body, for surgical sponges, wound dressings, feminine hygiene products and reuseable woven knit fabrics. Similarly, wettability is an important material surface property for printing and laminating.
Some success has been recently achieved by a glow discharge plasma treatment of meltblown polymer webs. The term "plasma" usually describes a partially ionized gas composed of ions, electrons and neutral species. This state of matter may be produced by the action of either very high temperatures, or strong direct current (DC) or radio frequency (RF) electric fields. High temperature or "hot" plasmas are represented by celestial light bodies, nuclear explosions and electric arcs. Glow discharge plasmas are produced by free electrons which are energized by an imposed DC or RF electric field and then collide with neutral molecules. These neutral molecule collisions transfer energy to the molecules and form a variety of active species including metastables, individual atoms, free radicals and ions. These active species are chemically active and/or capable of physically modifying the surface and may therefore serve as the basis of new chemical compounds and property modifications of existing compounds.
Low power plasmas known as dark discharge coronas have been widely used in the surface treatment of thermally sensitive materials such as paper, wool and synthetic polymers such as polyethylene, polypropylene, polyolefin, nylon and poly(ethylene terephthalate). Because of their relatively low energy content, corona discharge plasmas can alter the properties of a material surface without damaging the surface.
Glow discharge plasmas represent another type of low power density plasma useful for non-destructive material surface modification. These glow discharge plasmas can produce useful amounts of strong ultraviolet radiation. Glow discharge plasmas have the additional advantage therefore of producing UV radiation in the simultaneous presence of active species. However, glow discharge plasmas have heretofore been successfully generated typically in low pressure or partial vacuum environments below 10 torr. Several polymer species of meltblown webs exposed to low pressure glow discharge plasmas respond with enhanced surface wettability characteristics. However, the chemical/physical mechanism is not understood and the characteristic is lost upon drying. Rewettability remains elusive.
It is an object of the present invention, therefore, to provide a non-byproduct producing process for enhancing the wettability of meltblown polymer webs and any type of polymeric substrate.
Another object of the invention is to teach a glow discharge plasma process for treating meltblown polymer web that provides a stable, rewettable product.
Another object of the invention is to provide a method and apparatus for continuously processing a meltblown polymer web of indefinite length through a glow discharge plasma at atmospheric pressure and standard temperature.
It is, therefore, an object of the present invention to teach the construction and operating parameters of a glow discharge plasma having operability in an environmental pressure of about one atmosphere or slightly greater.